/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Append input mesh to our internal Mesh. If the internal Mesh has attributes enabled, per-triangle normals will be computed and set. No other attributes are initialized.
void AppendMesh ( const [TTriangleMeshAdapter](API\Runtime\GeometryCore\TTriangleMeshAdapter)< double > * AppendMesh, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMapsOut, [TFunction](API\Runtime\Core\GenericPlatform\TFunction)< FVector3d(int, const FVector3d &)> PositionTransform )